cmd/ld: when linking externally, only generate dwarf info for ELF
cmd/ld: when linking externally, only generate dwarf info for ELF

R=golang-dev, iant
CC=golang-dev
https://codereview.appspot.com/9025047

pombredanne/go-deleteme,apparentlymart/golang,dsriram/golang,pombredanne/go-deleteme,pombredanne/go-deleteme,gnanderson/go,apparentlymart/golang,dsriram/golang,pombredanne/go-deleteme,pombredanne/go-deleteme,gnanderson/go,gnanderson/go,pombredanne/go-deleteme,gnanderson/go,tv42/old-go,dsriram/golang,gnanderson/go,apparentlymart/golang,apparentlymart/golang,dsriram/golang,apparentlymart/golang,tv42/old-go,gnanderson/go,apparentlymart/golang,dsriram/golang,tv42/old-go,dsriram/golang,apparentlymart/golang,gnanderson/go,tv42/old-go,tv42/old-go,pombredanne/go-deleteme,tv42/old-go,tv42/old-go,dsriram/golang